A man convicted in the killing of Russian opposition politician Boris Nemtsov has been discharged from jail after signing a contract to join the military operation in Ukraine, state-run news agencies TASS and RIA Novosti reported on Saturday.

Nemtsov, a critic of President Vladimir Putin and former deputy prime minister under president Boris Yeltsin, was shot dead in 2015 as he walked across a bridge near the Kremlin in the heart of the Russian capital.

In 2017, a Russian court sentenced five men to prison terms ranging from 11 and 20 years for his murder. Among them was Tamerlan Eskerkhanov, who was convicted as an accomplice and jailed for 14 years.

"Eskerkhanov signed a contract with the defence ministry in March 2024, was pardoned, and then released from his penal colony," TASS cited a source in law enforcement agencies as saying.

"He went to one of the assault units and is now carrying out combat missions in the special military operation zone."

He added that the other convicts jailed over Nemtsov's killing were still in jail because they had refused to sign contracts with the military.

Ilya Yashin, Nemtsov's one-time spokesman who was freed last week in a high-profile prisoner exchange between Russia, Belarus, the United States and several European countries, called Eskerkhanov's release "scorn for memory of my dead friend."

Tens of thousands of Russian prisoners have volunteered to join the Russian army fighting in Ukraine, taking advantage of an offer of clemency for those who survive their stints at the front.

The recruitment of prisoners was initially pioneered by the Wagner mercenary group, whose leader Yevgeny Prigozhin was killed in an August 2023 plane crash after a failed mutiny against Russia's military leadership.

Russia's defence ministry has since adopted the tactic, forming its Storm-Z units partly out of convict volunteers recruited directly from prisons.